602 T. W. STAXTOX MESOZOIC OF MEXICO AND CENTRAL AMERICA 



work of Burckliardt, Bose. and other members of the Geological Institute 

 of Mexico, but many desired details are still lacking, especially concern- 

 ing southern Mexico, and the faunas and formations of the Central Amer- 

 ican States are known only in a general way. It is obvious that deduc- 

 tions from such incomplete data must Ije only tentative. Even in areas 

 where the stratigraphy, paleontology, and areal distribution are thor- 

 oughly known, there is usually an element of doubt and uncertainty in 

 interpreting sea and land connections. There is nearly always more than 

 one possible explanation of the facts observed, and the most obvious ex- 

 planation may not be the true one. A great difference between contem- 

 poraneous marine faunas such as is found between the Shasta fauna of 

 the United States Pacific coast and the Comanche faunas of Texas and 

 Mexico strongly suggests a persistent land barrier; but other conditions, 

 such as climate, character of bottom, etcetera, may be equally effective in 

 causing differences in faunas, and when the Comanche, or Gulf, type of 

 sediments and fauna seems to extend entirely across southern Mexico, 

 reaching the Pacific coast, it becomes necessary either to abandon the 

 land barrier or to shift it westward beyond the present continental area. 

 There is equal difficulty in establishing particular interoceanic connec- 

 tions. Close similarity of contemporaneous marine faunas with consid- 

 erable specific identity is good evidence of an open route for migration 

 between the two areas, but that route may not always have been the one 

 which noAv looks easiest. Certain of the Triassic and Jurassic faunas of 

 California show close relationship with the corresponding faunas of the 

 Mediterranean province in Europe, and their relationship has very natu- 

 rally been attributed to migration through the Central American portal, 

 as J. P. Smith has called it, but the possibility of an Indo-Pacific route, 

 or even of an Arctic route, should not be overlooked. 



With these general words of caution concerning the nature of the data 

 and the conclusions that may be placed upon them, we may proceed to 

 sketch the oscillations of land and sea in Mexico and Central America 

 during the Mesozoic era, beginning with early Triassic time. 



Triassic 



The Triassic was in general a period of emergence for the North 

 American Continent. Practically the whole of the continent was above 

 sealevel most of the time, and the minor recorded changes in strand-line 

 in the area north of Mexico affected only the Pacific coast. Marine 

 Lower Triassic deposits are knoAvn only in eastern California, Utah, and 

 southeastern Idaho, probably extending into western Wyoming. The 

 earliest fauna in these beds, the Meehocems fauna, is Asiatic in its affini- 



